Re: [PATCH v2 07/12] x86: add new features for paravirt patching

[Borislav Petkov <bp@xxxxxxxxx>]

On Fri, Nov 20, 2020 at 12:46:25PM +0100, Juergen Gross wrote:
> diff --git a/arch/x86/include/asm/cpufeatures.h 
> b/arch/x86/include/asm/cpufeatures.h
> index dad350d42ecf..ffa23c655412 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -237,6 +237,9 @@
>  #define X86_FEATURE_VMCALL           ( 8*32+18) /* "" Hypervisor supports 
> the VMCALL instruction */
>  #define X86_FEATURE_VMW_VMMCALL              ( 8*32+19) /* "" VMware prefers 
> VMMCALL hypercall instruction */
>  #define X86_FEATURE_SEV_ES           ( 8*32+20) /* AMD Secure Encrypted 
> Virtualization - Encrypted State */
> +#define X86_FEATURE_NOT_XENPV                ( 8*32+21) /* "" Inverse of 
> X86_FEATURE_XENPV */
> +#define X86_FEATURE_NO_PVUNLOCK              ( 8*32+22) /* "" No PV unlock 
> function */
> +#define X86_FEATURE_NO_VCPUPREEMPT   ( 8*32+23) /* "" No PV 
> vcpu_is_preempted function */

Ew, negative features. ;-\

/me goes forward and looks at usage sites:

+       ALTERNATIVE_2 "jmp *paravirt_iret(%rip);",                      \
+                     "jmp native_iret;", X86_FEATURE_NOT_XENPV,        \
+                     "jmp xen_iret;", X86_FEATURE_XENPV

Can we make that:

        ALTERNATIVE_TERNARY "jmp *paravirt_iret(%rip);",
                      "jmp xen_iret;", X86_FEATURE_XENPV,
                      "jmp native_iret;", X86_FEATURE_XENPV,

where the last two lines are supposed to mean

                            X86_FEATURE_XENPV ? "jmp xen_iret;" : "jmp 
native_iret;"

Now, in order to convey that logic to apply_alternatives(), you can do:

struct alt_instr {
        s32 instr_offset;       /* original instruction */
        s32 repl_offset;        /* offset to replacement instruction */
        u16 cpuid;              /* cpuid bit set for replacement */
        u8  instrlen;           /* length of original instruction */
        u8  replacementlen;     /* length of new instruction */
        u8  padlen;             /* length of build-time padding */
        u8  flags;              /* patching flags */                    <--- 
THIS
} __packed;

and yes, we have had the flags thing in a lot of WIP diffs over the
years but we've never come to actually needing it.

Anyway, then, apply_alternatives() will do:

        if (flags & ALT_NOT_FEATURE)

or something like that - I'm bad at naming stuff - then it should patch
only when the feature is NOT set and vice versa.

There in that

        if (!boot_cpu_has(a->cpuid)) {

branch.

Hmm?

>  /* Intel-defined CPU features, CPUID level 0x00000007:0 (EBX), word 9 */
>  #define X86_FEATURE_FSGSBASE         ( 9*32+ 0) /* RDFSBASE, WRFSBASE, 
> RDGSBASE, WRGSBASE instructions*/
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 2400ad62f330..f8f9700719cf 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -593,6 +593,18 @@ int alternatives_text_reserved(void *start, void *end)
>  #endif /* CONFIG_SMP */
>  
>  #ifdef CONFIG_PARAVIRT
> +static void __init paravirt_set_cap(void)
> +{
> +     if (!boot_cpu_has(X86_FEATURE_XENPV))
> +             setup_force_cpu_cap(X86_FEATURE_NOT_XENPV);
> +
> +     if (pv_is_native_spin_unlock())
> +             setup_force_cpu_cap(X86_FEATURE_NO_PVUNLOCK);
> +
> +     if (pv_is_native_vcpu_is_preempted())
> +             setup_force_cpu_cap(X86_FEATURE_NO_VCPUPREEMPT);
> +}
> +
>  void __init_or_module apply_paravirt(struct paravirt_patch_site *start,
>                                    struct paravirt_patch_site *end)
>  {
> @@ -616,6 +628,8 @@ void __init_or_module apply_paravirt(struct 
> paravirt_patch_site *start,
>  }
>  extern struct paravirt_patch_site __start_parainstructions[],
>       __stop_parainstructions[];
> +#else
> +static void __init paravirt_set_cap(void) { }
>  #endif       /* CONFIG_PARAVIRT */
>  
>  /*
> @@ -723,6 +737,18 @@ void __init alternative_instructions(void)
>        * patching.
>        */
>  
> +     paravirt_set_cap();

Can that be called from somewhere in the Xen init path and not from
here? Somewhere before check_bugs() gets called.

> +     /*
> +      * First patch paravirt functions, such that we overwrite the indirect
> +      * call with the direct call.
> +      */
> +     apply_paravirt(__parainstructions, __parainstructions_end);
> +
> +     /*
> +      * Then patch alternatives, such that those paravirt calls that are in
> +      * alternatives can be overwritten by their immediate fragments.
> +      */
>       apply_alternatives(__alt_instructions, __alt_instructions_end);

Can you give an example here pls why the paravirt patching needs to run
first?

Thx.

-- 
Regards/Gruss,
    Boris.

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